Is there somewhere that describes how to play the game a little bit? I ask because I didn't figure out what the cogs were for, and I didn't realise why it was important to smash everything to collect coins, until a couple of "levels" in, when I met the upgrade machine. I still don't know what the green cubes that sometimes drop when you kill bad guys are for.

As far as I can remember, this stuff isn't explained in game and you are meant to find out what all of these things do on your own. With cogs and gold it's easy because they are displayed as the currency for the upgrades. The small green cubes dropped by defeated enemies heal you a small amount.